It is known to provide one or more coded data structures on a surface that can be read and decoded by a suitable sensing device. Various embodiments of such a device incorporating an optical sensor are described in many of the documents incorporated into the present application by cross-reference.
The coded data structures disclosed in these documents include target features that enable the sensing device to identify the position of each structure. The relative positions of the features within each structure can also be interpreted to determine perspective distortion of the structure as sensed, enabling perspective correction to be performed on the sensed data. However, to enable the sensing device to decode the data in the structure, it is necessary that the rotational orientation of the structure be determined. Typically, this is achieved by providing at least one feature that is rotationally asymmetric in some way. For example, in one embodiment, a keyhole-shaped feature is provided that can be located with respect to the other features, and then recognised to ascertain the rotational orientation of the structure in relation to the sensing device. The actual data that is encoded in the data structure can then be decoded, since its position in the data structure can be inferred from the structure's position and rotational orientation.
Disadvantages with this arrangement include the need to dedicate space to one or more orientation features, and the difficulty of including redundancy in such features for the purposes of allowing rotational orientation determination in the presence of damage to the features. It is desirable, therefore, to encode orientation information both more space-efficiently and in an error-detectable and/or error-correctable fashion.